The never ending erosive effect of waves, tides and storms on docks, piers and shorelines has given rise to numerous methods of structure and reinforcement. Along the many miles of "waterfront" property now common in the coastal areas of the United States and other parts of the world, many types of structures have been used to contain and keep the land from slipping into the sea, and also provide a suitable pier for the mooring of boats and watercraft. Among the most common structure used is a continuous seawall made of sheetpile, which sheetpile is either concrete or wooden panels. These panels normally extend one to two feet below the existing water/sea-bed at the time of original installation. This type of seawall is normally topped with a continuous concrete pile cap which secures the top of the sheetpile, provides a usable walking surface and also is used as an anchor point and horizontal structural member for a metallic tie-back rod on the landside of the seawall. With one end anchored in the pile cap, the land end of the tie back rod is secured in a concrete thrust block which is buried. This type of seawall is normally reinforced on the inland side of the sheetpile with backfill material such as rip-rap, limestone or other rock and sand packing. This backfill material is then covered with grass sod to give a usable attractive surface. Over a period of time, cracks will develop where the sheetpile panels abut and tidal surges and wave action will filter through the seawall and begin to scour the backfill material behind the seawall and wash away the sand and fines packed around the rip-rap, limestone or other rock in the backfill material. The action of the water will also deposit the exfiltrate sand and fines scoured from the landside of the seawall on the seaside, thereby lessening the water depth and making the seawall unusable for mooring watercraft. The water action will also cause the cracks between the sheetpile panels to increase in size, thereby accelerating the scouring process. Because the water action scours the backfill material behind the seawall, voids are created between the rip-rap and limestone, causing it to settle along with the sod at ground level. This settling process also exposes the tie back rod.
Up to now, there are only several solutions to this problem and none of them are permanent, lasting no more than six weeks to a year. Some of these solutions are extremely costly. One of the present solutions is to backfill the voids created behind the seawall on land with limestone or gravel-aggregrate with sand and sometimes without sand. However, this method does not prevent further erosion of the fines beneath the aggregrate and is, at best, a temporary solution. Another method is to remove the backfill material such as rip-rap or limestone and sand to expose the landslide of the seawall for the installation of a sealing membrane. Once the membrane is installed the seawall is again backfilled with rip-rap or limestone and sand and resodded or reseeded as necessary. Although more costly than simply adding backfill as mentioned above, this again is not a permanent solution, simply because the sealing membrane is not installed far enough below the water level to prevent wave and tidal action from infilitrating and continuing to scour. A third method of dealing with the problem is to completely replace the sheetpiling and pilecap which is at least as expensive as the original installation, if not more so. Even after a complete replacement, the inherent problem remains.
In other coastal areas, certain regulatory agencies do not permit continuous sheetpile seawalls but will only allow rock revetment on the shoreline. Rock revetment is essentially the piling of heavy rocks, rip-rap, limestone or similar material at the shoreline to the desired level. Here again the ever constant wave and tidal action removes the sand and fines, resulting in the settling and movement of the rock revetment. This eventually undermines the integrity and frontal protection of the revetment type seawall.